Work machines such as, for example, on or off-highway haul or vocational trucks, construction equipment, and other work machines known in the art often include a multi-speed bidirectional transmission that has one or more hydraulically actuated friction clutches. These clutches may be selectively filled with pressurized fluid to produce predetermined output ratios of the transmission in either forward or reverse directions. The filling of the friction clutches may be controlled by electro-hydraulic proportional valve elements. That is, in response to an applied current the valve element moves to connect a source of pressurized fluid with the clutch to fill and thereby engage the clutch. As current is removed from the valve element, the fluid is drained from the clutch to disengage the clutch.
During operation of the work machine, it is possible for the transmission to lose electrical power. This loss of power typically results in draining of the friction clutches and loss of functionality of the work machine. Without intervention, the work machine could be inconveniently stranded away from service and repair resources. In addition, if the loss of electrical power occurs during an engine retarding operation, the ability to slow the work machine could be adversely affected.
One method of maintaining functionality of a transmission during an electrical power failure is described in U.S. Pat. No. 5,682,791 (the '791 patent) issued to Liesener on Nov. 4, 1997. The '791 patent describes a transmission having an independent latching system. Specifically, the transmission of the '791 patent includes a plurality of electro-hydraulic valve mechanisms. Each of the electro-hydraulic valve mechanisms includes a proportional valve element connected between a hydraulically actuated clutch, a source of pressurized fluid, and a low pressure reservoir to selectively fill and engage or drain and disengage the clutch. Each of the electro-hydraulic valve mechanisms also includes a latching shuttle valve disposed between the associated proportional valve element and the low pressure reservoir. A first end of the latching shuttle valve is in communication with the associated clutch.
The transmission of the '791 patent also includes a master latch valve connected between the source, the reservoir, and a second opposed end of each latching shuttle valve. In response to an electrical power failure, the master latch valve is spring biased to connect the reservoir with the second end of the latching shuttle valves. In this situation, the force generated by the pressurized fluid in the actuated ones of the hydraulically actuated clutches acting on the first end of the latching shuttle valves quickly moves the respective latching shuttle valves to block fluid from draining from the clutches to the reservoir.
Although the latching system of the '791 patent may sufficiently maintain functionality of the transmission during an electrical power failure, it may be expensive and difficult to package. In particular, because each of the electro-hydraulic valve mechanisms includes a latching shuttle valve, the cost of the '791 system may be high. In addition, the numerous valve elements of the '791 system may be difficult to package within an existing transmission or within the undercarriage space of a vehicle.
The disclosed transmission is directed to overcoming one or more of the problems set forth above.